In general, inkjet printing machines or printers include at least one printhead unit that ejects drops of liquid ink onto recording media or an imaging member for later transfer to media. Different types of ink may be used in inkjet printers. In one type of inkjet printer, phase change inks are used. Phase change inks remain in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The printhead unit ejects molten ink supplied to the unit onto media or an imaging member. Once the ejected ink is on media, the ink droplets quickly solidify.
The media used in both direct and offset printers may be in web form. In a web printer, a continuous supply of media, typically provided in a media roll, is entrained onto rolls that are driven by motors. The motors and rolls pull the web from the supply roll through the printer to a take-up roll. The rollers are arranged along a linear media path, and the media web moves through the printer along the media path. As the media web passes through a print zone opposite the printhead or heads of the printer, the printheads eject ink onto the web. Along the feed path, tension bars or other rolls remove slack from the web so the web remains taut without breaking.
Existing web printing systems use a registration control method to control the timing of the ink ejections onto the web as the web passes the printheads. One known registration control method that may be used to operate the printheads is the single reflex method. In the single reflex method, the rotation of a single roll at or near a printhead is monitored by an encoder. The encoder may be a mechanical or electronic device that measures the angular velocity of the roll and generates a signal corresponding to the angular velocity of the roll. The angular velocity signal is processed by a controller executing programmed instructions for implementing the single reflex method to calculate the linear velocity of the web. The controller may adjust the linear web velocity calculation by using tension measurement signals generated by one or more load cells that measure the tension on the web near the roll. The controller implementing the single reflex method is configured with input/output circuitry, memory, programmed instructions, and other electronic components to calculate the linear web velocity and to generate the firing signals for the printheads in the marking stations.
Another existing registration control method that may be used to operate the printheads in a web printing system is the double reflex method. In the double reflex method, each encoder in a pair of encoders monitors one of two different rolls. One roll is positioned on the media path prior to the web reaching the printheads and the other roll is positioned on the media path after the media web passes the printheads. The angular velocity signals generated by the two encoders for the two rolls are processed by a controller executing programmed instructions for implementing the double reflex method to calculate the linear velocity of the web at each roll and then to interpolate the linear velocity of the web at each of the printheads. These additional calculations enable better timing of the firing signals for the printheads in the marking stations and, consequently, improved registration of the images printed by the marking stations in the printing system.
Moving the web through the media path in a controlled manner presents challenges to web printing systems. If the web slips when engaged with one or more rolls in the media path, the position of the media web with respect to the printheads is affected and errors in images formed on the media web may occur. Media slippage may cause errors between the actual velocity of the web and the web velocity that is identified with respect to the angular velocity of the rolls, reducing the effectiveness of single and double reflex registration techniques. Increasing tension at a roll is known to increase friction between the roll and media web and reduce the likelihood of the media web slipping on the roll. Too much tension, however, can break or distort the media web, resulting in lost productivity when the printer is unable to print to the media web. In existing printers, different printer configurations and media types may have known tension settings that enable the media web to move through the media path without slipping or breaking. During operation, however, the mechanical tolerances and frictional coefficients of various printer components may change, and the known tension settings may no longer be suitable. Thus, improvements in operating continuous web printing systems to enable accurate reflex registration control would be beneficial.